Light is absorbed by plants and forms the basis of most food chains on Earth. Photons from the sun are absorbed by plants to convert carbon dioxide and water into carbohydrates. Photosynthesis is a complex multi-step chemical reaction that is powered by photons of specific wavelengths or energies. While sunlight has a broad spectrum that appears white, plants appear green since they reflect green and absorb light of other colors. Three principal characteristics of light affect plant growth: quantity (intensity or photon quantity), quality (light wavelength or color), and duration (time).
Plants and animals are sensitive to light for a variety of reasons that are not fully understood. While photosynthesis strongly absorbs red light, there is some absorption of other colors. Leafy plants also absorb blue light, which promotes photomorphogenesis, phototropism, and flowering.
FIG. 1 is a graph of absorption of light by a plant as a function of wavelength. Short wavelength light, such as blue in the 400-500 nm range, provide photons that are absorbed by chlorophyll and promote plant growth. While most green light is reflected by plants, some green or yellow light is still absorbed. The absorption curve has a low trough in the 500-600 nm wavelength region.
The largest absorption by the plant occurs in the 600-700 nm red region. photosynthesis strongly absorbs red light. Red light promotes seed germination, pigment formation, flowering, and may induce dormancy.
Another peak in absorption occurs in the far-red region of 700-750 nm. Far-red wavelengths influence seed germination, flowering, and stem elongation (plant height).
The exact shape of the curve of plant absorption shown in FIG. 1 varies with the species of plant, and perhaps even with the growth stage that the plant is in.
Altering the wavelengths of light applied to a plant may improve targeted results. For example, increasing the intensity of red wavelengths may be better for a fruit tree or a flower, while green lettuce may benefit from increased blue light. Applying increased far-red light to plants in a nursery may increase the height of these plants, improving their salability and profit.
FIG. 2 shows spectra of prior-art lights compared with a plant sensitivity curve. A Plant Sensitivity Curve (PSC), such as the one shown in FIG. 1, is overlaid on the graphs as a dashed line. This PSC has a strong absorption in the longer red wavelengths and in the shorter blue wavelengths than in the middle yellow-green wavelengths. Every plant or organism has a different PSC.
Traditional plant lighting systems use High-Pressure Sodium (HPS) or Metal Halide (MH) bulbs. These light sources produce wavelengths of light that depend on chemicals such as sodium that emit light at specific wavelengths when excited. A HPS source emits a red-orange spectrum with a red peak energy that is near to the red peak of the plant absorption as shown by the dashed PSC (but little in the Far Red region). HPS lights are often used for fruiting and flowering plants that need red light.
Metal Halide (MH) lights emit a blue-white light that drops off in the red region where the PSC has a broad peak. MH lights may be better suited for vegetative growth and seedlings but are less effective for fruiting and flowering.
Both MH and HPS lights do not exactly match the plant's absorption. Both have peaks in the green-yellow region between 520-610 nm. About 20 to 30% of the light is emitted outside the Photosenthetically Active Region (PAR). Thus the spectral efficiency is lacking. Energy that is not absorbed by the plant is wasted.
Both MH and HPS light sources produce a significant amount of waste heat. This heat can burn or otherwise damage plants, so the lights must be kept a safe distance from the plants. However, as the distance is increased, the intensity of light reaching the plant decreases with the square of the distance. The greenhouse may require cooling equipment to remove the waste heat from these lights, increasing energy consumption and cost. These lights cannot be switched on and off frequently to mimic secondary effects such as sun fleck (brief increases in solar irradiance), such as occur naturally when plants on a forest floor are shaded by the movement of leaves on trees above blowing in the wind. When HPS or MH lights are switched on and off frequently, their ballasts may burn out and fail.
More recently, Light-Emitting Diode (LED) lamps have become available. LEDs are more efficient since LEDs have a higher efficacy, (optical watts)/(input energy watts). LEDs can reduce power consumption compared to HPS and MH lamps. Radiated heat is significantly reduced, so the LED lamps can be placed closer to the plants, increasing the light energy that reaches the plant.
Different types and species of plants may have different lighting requirements. These light requirements may vary with the different stages of growth. While much research has been done to study the effects of different wavelengths of light upon plants, the results vary widely and are hard to duplicate, especially since they are influenced by a number of variables. Commercial interests such as commercial growers could improve yield efficiency if they could find light research results for their particular plants or could better understand the relationship between lighting and plant yields.
What is desired is a programmable grow light using LED's of different wavelengths. A programmable grow light that can adjust the wavelengths of light reaching a plant, and can vary these wavelengths over different growth phases of a plant's life is desirable. A host interface that can record these wavelengths in a light map that can be uploaded and shared with others on a web site is desirable. A voting, ranking, or evaluation mechanism of these light maps is desirable to allow a large on-line community to discuss, modify, and recommend different wavelength settings resulting in optimized light maps for different species of plants.